age -o follows a symlink and writes a pipe or device directly (closes #59)

age encrypt -o and age decrypt -o always wrote a new file beside the
named path and renamed it over that path, which replaced a symlink, a
named pipe or a device such as /dev/null with a regular file and made
-o /dev/stdout fail. The path is now looked at without following a
final symlink: a missing path or a regular file is replaced by rename
as before, a symlink gets the same treatment for what it points at,
and anything else is written to directly, without catching signals.
A symlink that points at nothing is refused. The README says so, and
that a replaced file has mode 0600.

Model: opus-5-5

README.md

@@ -250,11 +250,17 @@ identity's own recipient, plus any given with `--to`, so the same mnemonic can
 always decrypt what it encrypted. Output goes to `-o` or standard output;
 `--armor` writes the text form. Nothing is written except the output.
 
+A regular file already at the `-o` path is replaced, and the new file has mode
+`0600`. A symlink there is followed, and what it points at is treated the same
+way, so the link keeps pointing where it did; a symlink that points at nothing
+is refused. A named pipe or a device, such as `/dev/null` or `/dev/stdout`, is
+written to directly.
+
 ### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
 
 Decrypts the file (or standard input) with the derived identity. Output goes to
-`-o` or standard output. If the identity is not one of the recipients, the tool
-says so and exits with status 1.
+`-o`, which is treated as for `encrypt`, or standard output. If the identity is
+not one of the recipients, the tool says so and exits with status 1.
 
 ## Derived mnemonics: `keyfunc mnemonic`
 
@@ -291,15 +297,17 @@ passes through `ssh`'s own exit status.
 
 SIGINT, SIGTERM and SIGHUP end any command at once, at the mnemonic prompt too,
 with the status a shell gives a program killed by that signal (130 for SIGINT).
-While `age encrypt -o` or `age decrypt -o` is writing the file, the signal makes
-it remove the unfinished file, leave a file already at the named path as it was,
-and exit with status 1. That holds for a signal that has reached `keyfunc` when
-its input ends; a later one leaves the whole file in place. Ctrl-C on a pipeline
-ends the input at the same moment, and on Linux `keyfunc` sees the signal first,
-though no system promises that. While `ssh to` or `ssh install` has `ssh` or
-`sftp` running, the signal ends that program instead, the tool removes its agent
-socket or working files, and it exits with status 1, or for `ssh to` with
-`ssh`'s own status if `ssh` reported one.
+While `age encrypt -o` or `age decrypt -o` is writing a new file or replacing a
+regular one, the signal makes it remove the unfinished file, leave a file
+already at the named path as it was, and exit with status 1. That holds for a
+signal that has reached `keyfunc` when its input ends; a later one leaves the
+whole file in place. Ctrl-C on a pipeline ends the input at the same moment, and
+on Linux `keyfunc` sees the signal first, though no system promises that. A
+named pipe or a device at the `-o` path is written to directly, and the signal
+ends the tool there as it ends any other command. While `ssh to` or
+`ssh install` has `ssh` or `sftp` running, the signal ends that program instead,
+the tool removes its agent socket or working files, and it exits with status 1,
+or for `ssh to` with `ssh`'s own status if `ssh` reported one.
 
 ## Entrypoints
 

internal/cli/age/age.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -180,7 +181,78 @@ func input(cmd *cobra.Command, args []string) (io.Reader, func(), error) {
 	return file, func() { _ = file.Close() }, nil
 }
 
-// output has the work write a new file beside the named one, and puts
+// output has the work write to the named path, going by what is there
+// without following a final symlink:
+//
+//   - nothing, or a regular file: replace writes a new file beside it
+//     and renames that over it;
+//   - a symlink: the same for what it points at, so that the link keeps
+//     pointing where it did; one that points at nothing is refused;
+//   - anything else, such as a named pipe or a device like /dev/null:
+//     direct writes to it, since a rename would put a regular file in
+//     its place.
+func output(
+	name string, src io.Reader, work func(io.Writer, io.Reader) error,
+) error {
+	info, err := os.Lstat(name)
+
+	switch {
+	case errors.Is(err, fs.ErrNotExist):
+		return replace(name, src, work)
+	case err != nil:
+		return fmt.Errorf("looking at %s: %w", name, err)
+	case info.Mode().IsRegular():
+		return replace(name, src, work)
+	case info.Mode().Type() == fs.ModeSymlink:
+		// os.Stat follows the link as opening it would. /dev/stdout
+		// needs that: it reaches a pipe or a terminal through a link
+		// that names no path.
+		info, err = os.Stat(name)
+		if err != nil {
+			return fmt.Errorf("following %s: %w", name, err)
+		}
+
+		if !info.Mode().IsRegular() {
+			return direct(name, src, work)
+		}
+
+		target, err := filepath.EvalSymlinks(name)
+		if err != nil {
+			return fmt.Errorf("following %s: %w", name, err)
+		}
+
+		return replace(target, src, work)
+	default:
+		return direct(name, src, work)
+	}
+}
+
+// direct has the work write straight to the named path, which is there
+// and is not a regular file. No signal is caught, so one ends the tool
+// as it ends any other command.
+func direct(
+	name string, src io.Reader, work func(io.Writer, io.Reader) error,
+) error {
+	file, err := os.OpenFile(name, os.O_WRONLY, 0) //nolint:gosec // the -o path
+	if err != nil {
+		return fmt.Errorf("opening %s: %w", name, err)
+	}
+
+	failed := work(file, src)
+	closeErr := file.Close()
+
+	if failed != nil {
+		return failed
+	}
+
+	if closeErr != nil {
+		return fmt.Errorf("finishing %s: %w", name, closeErr)
+	}
+
+	return nil
+}
+
+// replace has the work write a new file beside the named one, and puts
 // the new file in the named file's place only when the work succeeded,
 // so a file that is already there survives a run that failed.
 //
@@ -189,7 +261,7 @@ func input(cmd *cobra.Command, args []string) (io.Reader, func(), error) {
 // new file is removed and ErrInterrupted returned, at once if the work
 // is still running, without waiting for it, since it may be blocked
 // reading its input.
-func output(
+func replace(
 	name string, src io.Reader, work func(io.Writer, io.Reader) error,
 ) error {
 	// received is registered before the context, so it gets every

internal/cli/age_test.go

@@ -3,6 +3,7 @@ package cli_test
 import (
 	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -98,6 +99,58 @@ func TestARefusedDecryptionLeavesTheOutputFileAlone(t *testing.T) {
 	require.Equal(t, "what was already there\n", string(kept))
 }
 
+func TestASymlinkAtTheOutputPathStaysAndItsTargetGetsTheOutput(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	plain := written(t, "notes.txt", "the secret\n")
+	target := written(t, "notes.age", "what was already there\n")
+	link := filepath.Join(t.TempDir(), "notes.age")
+	require.NoError(t, os.Symlink(target, link))
+
+	run(t, "age", "encrypt", "-o", link, plain)
+
+	pointsAt, err := os.Readlink(link)
+	require.NoError(t, err)
+	require.Equal(t, target, pointsAt)
+	require.Equal(t, "the secret\n", run(t, "age", "decrypt", target))
+}
+
+func TestANamedPipeAtTheOutputPathIsWrittenToAndStaysAPipe(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	plain := written(t, "notes.txt", "the secret\n")
+	pipe := filepath.Join(t.TempDir(), "notes.age")
+	require.NoError(t, syscall.Mkfifo(pipe, fileMode))
+
+	// Opening the pipe to read waits until the tool opens it to write.
+	var sealed []byte
+
+	finished := make(chan error, 1)
+
+	go func() {
+		var err error
+
+		sealed, err = os.ReadFile(pipe) //nolint:gosec // the test's own path
+		finished <- err
+	}()
+
+	run(t, "age", "encrypt", "-o", pipe, plain)
+
+	select {
+	case err := <-finished:
+		require.NoError(t, err)
+	case <-time.After(5 * time.Second):
+		t.Fatal("nothing was written to the pipe")
+	}
+
+	info, err := os.Lstat(pipe)
+	require.NoError(t, err)
+	require.Equal(t, fs.ModeNamedPipe, info.Mode().Type())
+
+	sealedFile := written(t, "notes.age", string(sealed))
+	require.Equal(t, "the secret\n", run(t, "age", "decrypt", sealedFile))
+}
+
 func TestASignalStopsAnEncryptionAndLeavesNoFile(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())
 

internal/cli/cli.go

@@ -85,7 +85,8 @@ func init() {
 // signals to clean up first: "ssh to" and "ssh install" while they
 // have ssh or sftp running, so the child ends and their own cleanup
 // still runs, and "age encrypt -o" and "age decrypt -o" while they
-// write, so the unfinished file is removed.
+// write a new file to rename over the named one, so the unfinished file
+// is removed.
 func Main() int {
 	err := Root().Execute()
 	if err == nil {